Interesting article:
WHY FOUR GOSPELS?
I am saying that some of the Gospels contain the same passages verbatim, indicating that the Gospel-writers used the same sources (such as a purported source documenting Jesus' sayings; the Gospel writers incorporated these sayings into a narrative. Matthew and Luke used Mark as a source, but that doesn't mean that they "plagiarized", only that Mark served as a guideline).
Was the Gospel of Mark Written First?
The theory that Mark’s Gospel was written first and served as a source for Matthew and Luke is not based on “one logically cast-iron argument,” admits The Anchor Bible Dictionary. Yet, many scholars feel that Mark wrote his Gospel before Matthew and Luke wrote theirs because, they claim, Mark adds little to the other Gospels. For example, 19th-century Bible scholar Johannes Kuhn insisted that Mark’s Gospel must have been written first. Otherwise, Kuhn said, “one would have to imagine that Mark had cut the two scrolls of Matthew and Luke up into little snippets, mixed these together in a pot, and produced his Gospel from this mixture.”
Since Mark’s Gospel is the shortest, it is not surprising that it contains the least amount of unique material. Still, that does not prove that it must have been written first. Further, it is simply not true that Mark adds nothing to Matthew and Luke. In Mark’s vigorous, fast-moving account of Jesus’ ministry, there are actually more than 180 passages and fascinating details that are not found in Matthew and Luke, making it a truly unique account of Jesus’ life.—See box on page 13.
What About Document Q?
What can be said about document Q, which some claim was a source for Matthew and Luke? James M. Robinson, professor of religion, states: “Q is surely the most important Christian text that we have.” That statement is surprising because document Q does not exist today, and in reality, no one can prove that it ever existed! Its total disappearance is all the more remarkable because scholars claim that several copies of the document must have circulated. In addition, document Q is never quoted by the Church Fathers.
Think about this. Q is supposed to have existed and to have supported the hypothetical priority of Mark’s Gospel. Is that not a case of one hypothesis built upon another hypothesis? When it comes to theories such as these, we are wise to keep this proverb in mind: “A simple man believes every word he hears; a clever man understands the need for proof.”—Proverbs 14:15, The New English Bible.
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Continuing the illustration, the tax man wants to appeal to people of a Jewish background, so he groups some teachings or events in a way to help that primary audience. The physician highlights the healing of the sick or crippled, so he omits some things that the tax man recorded or presents them in a different order. The close friend emphasizes the teacher’s feelings and qualities. The younger man’s account is briefer, more succinct. Still, each man’s account is accurate. This well illustrates how having all four accounts of Jesus’ life enriches our understanding of his activities, teachings, and personality.
People may speak of ‘the Gospel of Matthew’ or ‘John’s Gospel.’ That is not inaccurate, for each contains “good news about Jesus Christ.” (Mark 1:1) However, in a larger sense, there is but one overall gospel, or good news, about Jesus​—available to us in the four records....]
Benedict Arnold 